A number of protein expression systems have been utilized to produce useful heterologous proteins. Escherichia coli is one of the most widely used hosts. While high levels of expression in bacterial systems are common, problems of improper folding and lack of post-translational processing often lead to functionally inactive proteins. The baculovirus expression system can be used to circumvent these problems. The baculovirus system enjoys many advantages for expressing a heterologous protein. For example, heterologous cDNA is expressed well although genes having introns are expressed less efficiently. Baculovirus expression of heterologous genes permits folding, post-translational modification, and oligomerization in manners often identical to those in vertebrate cells, e.g., mammalian cells. Further, proteins may be secreted from cells or targeted to different subcellular locations. Single polypeptide, dimeric and trimeric proteins have also been expressed in baculovirus systems. In addition, high level protein expression can be achieved using the strong polyhedrin promoter. Despite these advantages, baculovirus systems are still not satisfactory compared with vertebrate cell-based systems in several aspects. Examples include inefficient secretion from cells, improperly protein folding, and intracellular protein aggregation. Also, N-linked glycosylation sites are often either fully glycosylated or not glycosylated at all, as opposed to various glycoforms in mammalian cells. Further, species- or tissue-specific modifications are unlikely to occur in baculovirus expression systems. Mammalian cell-based expression systems generally do not have the problems seen in baculovirus expression systems. However, protein expression levels are often not satisfactory.